1. Field of the Invention
The invention relates to electric field induced electron emitters and more particularly, to field emission cathodes and related micro-electronic devices and field-emission displays based on the use of carbon-metal diamond-like composites materials.
2. Background Art
The interest to advanced field emission (FE) cathode materials arises from a number of applications including field emitters for microelectronic devices, flat panel displays, high intensity electron beams for accelerators and free electron lasers, and high current density cathodes for microwave power tubes, klystrons and gyrotrons. As compared with more conventional thermionic electron sources, cold FE cathodes require no heater circuit and are capable of generating high current densities.
The most common approach in concentrating electric field to create the field induced electron emission is fabrication of metal tips using geometry dependent vapor deposition, commonly termed as the Spindt type cathodes. Two important limitations of micro-fabricated field emitter tips are their poor reliability and stability. In large part, these limitations can be traced to the inherent chemical/thermodynamic instability of clean, highly curved surfaces. Conventional fabrication processes, combined with the usual operating conditions inevitably lead to the build up of contaminants on emitter surfaces acting as barriers for electrons resulting in formation of a large effective work function. Another problem with the tip cathodes originates from a mechanical damage of the tips by positively charged ions, which reduces the cathode lifetime.
A significant break-through occurred recently in the development of diamond film cold cathodes. Diamond thin films (including amorphous films) have been found to yield significant current densities with quite low fields. Amorphous diamond thin films are capable of emitting electrons at the electric field of less than 20 V/m. A current density as high as 100 mA/mm2 was achieved. It is agreed, that this material when used in FE displays (FED), has the potential for surpassing all other materials in terms of brightness, contrast, response time and low power consumption. It has been noted that CVD diamond films have two important properties that are favorable to cold, low-field emission, namely, their negative electron affinity in some crystallographic directions and presence of graphite inclusions. These inclusions play an important role of conductive channels to localize applied electrical field. However, during cathode operation, graphite inclusions appear to act as nucleation sites to induce further conversion of diamond to graphite, which leads to material degradation. In addition, it is very difficult to attain uniform emission with minimum cathode series resistance, which is important for application of this technology to high luminance/large format displays. Comparative analysis of the forgoing materials is represented in the Table.
Recently, a new approach based on carbon nano-tubes has been applied to FED fabrication. The carbon filaments stemming from the carbon film represent good electric field concentrators yielding emission threshold as low as 1 V m. The drawbacks of this approach are that the technology is non-planar and cannot use the photolithography processing. The resultant control voltages are typically in the range of hundreds voltage, which makes difficult utilizing this technology for micro-devices.
Another direction of the field emission cathodes and Field Emission Displays relies on fabrication of the cathodes in the shape of edges of thin films. Such an approach has significant advantages over the tip-based technology. First, the cathode edge can be made within planar technology, which dramatically reduces the fabrication cost. Second, the edge is much more resistive to a mechanical damage. Finally, the edge length can be made extremely long for each cathode thereby providing a high emission current.
Typical design of the edge-emitter diode is shown in FIG. 1 (see A. Kastalsky, et al, SID-2001, p.201). The cathode edge plane is above the anode electrode. The latter is placed into the well etched in the substrate. Vertical distance between cathode and anode planes, which can be made less than a micron, is the active gap, controlling the emission process. In the cited work, a thin carbon film was utilized as the edge emitter used for realization of the FED, with phosphor layer deposited on the anode electrode. In application to the micro-devices, such a design is particularly attractive since the electrodes are shifted from each other laterally, thus minimizing the device capacitance. Deposition of the nano-composite cathode layer is expected to reduce the emission threshold voltage down to 1 V/m or lower.
From the afore said, it is clear that for the edge emission devices, availability of a thin, conductive, thermally, chemically and mechanically stable emissive film is of a paramount importance.